Optical telescopes use the same part of the electromagnetic spectrum in
which the human eye operates. The larger the aperture, the more light is
collected and the more objects you can see. Astronomers often want to see
objects which are very faint so their telescopes are often very large.

Optical astronomical observatories are often located on the tops of hills.
One reason for this is to get above the clouds which block out the visible
light from stars and prevent observations. Sometimes they are sent into
orbit around the Earth to get above the fluctuations in the atmosphere
which can also make observations more difficult.